'ring': spread round the whole circle between the two radii (the
asteroid belt, the Kuiper belt) — drawn with a soft glow disc under
its points.
'shell': spread in every direction (the Oort cloud) — points only.
'cluster': concentrated in one or more local patches rather than round
the whole circle (e.g. asteroid clusters at a Lagrange point) — points
only, since a full-circle glow disc would draw a belt that isn't there.
The overall shape of a belt's population: